Had a few friends who were going through this on Windows, too. Their issue ended up being the unit frames addon they had was tainting the PvP menu, and somehow that hosed framerate in PvE raids, too.

This also happens on the Mac. Make sure you get a 5.2 unit frame mod if you use one.

Either way, that's not the issue I was having, but the hotfix server side yesterday fixed my major issue. I no longer nosedive in fps. I do dip a lot more than in patch 5.1, often going down to 20 fps in fights I stayed over 40 before. It's still playable, where the initial drop was to 0 or 1 fps, which was not playable.

I don't use any unit frame mods, so I don't think that's it for me, but I have noticed something curious. My lag spikes seem to come not due to graphical effects, but instead synched up with bosses using certain abilities. A lot of times the two are tied together -- for instance, I was getting a burst of lag every time Lei Shi used a new ability on LFR on Wednesday -- but it happens even with low-intensity abilities -- for instance, I was reliably getting a lag spike while tanking the Echo of Kros on the Isle of Thunder earlier today, every time he summoned a Skumblade Scavenger to eat. Ghost Dinosaur Stampede, which seems more graphically intensive, gave me no problems whatsoever.

I'll see if I can pick out any more abilities that trigger lag next time I raid.